In the recent past our society has seen an increasing and continuing computerization of many different types of information. Witness, for example, the amazing increase in digitizing analog signals of various types for storage, transmission and processing purposes. This trend is seen in all fields of electronics, particularly communication, process control and data processing. One particularly successful method of digitally describing analog signals has been what is termed in the art as pulse code modulation (PCM). This method involves sampling the analog signal at regular intervals and coding the signal amplitude in a digital format. A variation of this method includes comparing successive samples of the digital amplitude and storing or transmitting only the differences. This latter method is termed differential PCM.
A very special form of differential PCM involves digitally encoding analog signals by a process known as delta modulation. Analog-to-digital conversion using delta modulation techniques involves periodical comparisons of the analog signal to a reference signal. The instantaneous polarity of the difference between these two signals is sampled at intervals controlled by a clock to produce a logic ONE or ZERO. The sample is temporarily stored in a storage device (typically a flip-flop) where it is held until updated by a subsequent sample. The output of the storage device, which is also the output of the delta modulator, is applied to an integrating network which, through step-like increments, constructs the analog signal to form the reference signal that is compared to the original.
At the receiving or decoding end of the system, the digitized signal is received and applied to a similar integrating network to reconstruct and retrieve the analog signal in a representation that is as accurate a representation of the original as possible.
The particular conversion method chosen for digitizing (and subsequent recovery of) any particular analog signal requires several trade-offs. For example, if the particular analog signal to be digitized has a relatively wide dynamic range (e.g., the signal contains large amplitude variations) one can choose a PCM technique. However, PCM techniques utilize very high bit rates which, in turn, produce such disadvantages as the requirement of a much larger storage space to store digitized information. Moreover, PCM techniques operate using word configurations; that is, the information is in the form of successive, multi-bit data words. Accordingly, if some bits of information, the more significant or heavily weighted bits, are lost during transmission a significant amount of information can be lost. Finally, retrieval of an analog signal encoded using PCM techniques typically requires sophisticated filtering--adding to the expense and complexity of the method. Furthermore, many PCM systems are complex, expensive, and difficult to maintain.
On the other hand, one may choose to digitize the analog signal using a form of delta modulation and obtain the advantage attendant therewith of low bit rates. However, because of the restricted dynamic range per step of typical delta modulation systems, the difference between two adjacent sample values will be incorrect if the signal variation is large. The delta modulator system cannot, therefore, follow this change and the error is referred to as "slope overload". Thus, the fidelity of the signal reconstructed at the decoding or receiving end will not be quite what is desired.
One attempt to solve this particular problem has been to increase the quantizing steps of the delta modulator system by increasing the bit rate. Unfortunately, this solution is possible only at the expense of increased quantization noise (granular noise) that is introduced when the digitally encoded signal is converted back to analog form at the receiving or decoding end of the system. This distortion is experienced when the input analog signal has a negligible input change relative to the step size of the binary signal transmitted by the delta modulator. In trying to reproduce the input analog signal the receiver, therefore, introduces an overriding square wave or triangular wave pattern, depending upon whether or not it is a wide pulse or a narrow pulse system, because the step size is too large.
Other attempts to achieve as universal a conversion system as possible have included, for example, the use of a more specialized differential pulse code modulation method with more than one step size. This type of system is generally referred to as multilevel quantization. However, in addition to being more complex and expensive than simple delta modulators, such multilevel quantization systems are considerably more difficult to maintain and adjust.
Accordingly, there is a need to provide a low-cost, simple system that can digitize, transmit or store, and retrieve an analog signal of relatively wide dynamic range utilizing relatively low sample rates at relatively low bit rates.